Intrinsic Inhibitory Mechanisms of Vagal Nociceptors

Project Summary/Abstract In visceral inflammatory disorders the excitability and activation of visceral nociceptive sensory nerves are abnormally enhanced, contributing to the difficult-to-treat clinical symptoms such as visceral pain and discomfort, bladder irritability, diarrhea/constipation, cardiac arrhythmias, bronchospasm, heartburn, dyspnea, and unproductive coughing. The pathogenesis of visceral nociceptor sensitization is complex and poorly understood. Our long-term objectives are to elucidate the ionic/molecular mechanisms underlying the regulation and dysregulation of visceral nociceptor excitability, and to identify novel therapeutic strategies aimed at reducing the suffering of the millions afflicted with chronic visceral diseases. The excitability of neurons, including visceral nociceptors, is dynamically regulated by concerted action of excitatory and inhibitory ion channels. The last few decades of intensive research in visceral nociceptive nerves has mainly focused on various excitatory ion channels while much less is known about the K+ channels that exert inhibitory control on nociceptor excitability. Our recently published work and new preliminary data are revealing that among the plethora of functionally diverse voltage-gated potassium (KV) channels two subtypes, the KV7/M-type (IM) and KV1/D-type (ID) K+ channels, mediate the majority of sustained inhibitory K+ currents at the subthreshold voltages, thereby playing the critical role of "braking" the activation in vagal visceral nociceptors. We therefore hypothesize that inhibition in activity and/or expression of KV7/IM or KV1/ID channels in vagal nociceptors leads to vagal visceral nociceptor sensitization and thus contributes to exaggerated visceral nociceptive reflex responses, and that selective pharmacological activation of "braking" K+ channels may provide novel therapeutic strategies for the management of debilitating neurological symptoms associated with various visceral disorders. Our four specific aims are designed to address these hypotheses by investigating the consequences of genetic inactivation and pharmacological activation of KV7/IM and KV1/ID channels on the excitability of the vagal visceral nociceptive nerves at the levels of cell bodies, nerve terminals as well as defensive reflex behaviors, and by investigating the effects of visceral inflammation on the expression and function of these channels in vagal nociceptors. To achieve these aims, we will use the most up-to-date innovative technologies, including single-cell RNA sequencing, state-of-the-art patch clamp techniques, two- photon microscopic calcium imaging and extracellular electrophysiological recording of single fiber activities, and respiratory reflex measurement in awake mice, combined with sensory neuron- and nodose neuron- specific gene knock-out and knock-down strategies. By pursuing the proposed study, we will advance our knowledge about the ion channel mechanisms underlying the vagal nociceptor sensitization associated with inflammatory diseases, and identify the braking K+ channels as promising novel therapeutic targets to quell the abnormal visceral nociceptive responses in diseases.